The present invention generally relates to an X-Y table for precisely positioning a work and a tool incorporated in a machine tool, for example, and in particular, to an X-Y table employing a linear motor as a drive source therefor
A typical conventional type of X-Y table has heretofore been arranged such that a table body mounted on a base through an intermediate saddle can slide in both the X- and Y-axis directions, orthogonal to each other in rectangular coordinates. The feed mechanism of the X-Y table has been constituted by a combination of ball screw and nut assemblies and rotary motors such as a servomotor or stepping motor.
More specfically, a ball screw shaft is rotatably disposed on the upper side of the base so as to extend in an X-axis direction, and a ball nut, which is screwed onto the screw shaft, is secured to the intermediate saddle. A rotary motor is operatively connected to one end of the ball screw shaft so that the intermediate saddle is fed in the X-axis direction along the ball screw shaft by the rotation of the motor. Similarly, a ball screw shaft is disposed on the upper side of the intermediate saddle so as to extend in the Y-axis direction perpendicular to the longitudinal axis of the first ball screw shaft, and a ball nut screwed onto the second ball screw shaft is secured to the lower side of the table body. A rotary motor is operatively connected to one end of the ball screw shaft so that the table body is fed in the Y-axis direction relative to the intermediate saddle by the rotation of the corresponding motor.
In the above-described prior-art X-Y table, however, if the ball screw and nut assemblies are reduced in pitch, the table body can be precisely fed, but the feeding speed thereof unavoidably becomes slow. Otherwise, the table body can be fed at a high speed by increasing the rotational speed of each of the ball screws. The high speed rotation of the ball screw, however, involves a problem such that the rotational speed of each of the ball screws should be limited below a dangerous level. There is a case where the ball screw is twisted by the rotational torque of the rotary motor or where a backlash occurs between a ball screw and a ball nut, thereby causing mechanical error and thus resulting in an unfavorable low degree of accuracy in the positioning of the table body.
The X-Y table of conventional type further provides disadvantages as described below.
The location of the rotary motors, ball screw and nut assemblies and so forth on a movable section such as the intermediate saddle or the table body makes heavy the movable section and large the inertia force thereof. Thus, every time the table body is stopped, the stop position tends to shift under the influence of this inertia force, so that the precision of positioning gradually deteriorates.
Since the table body and other members are disposed on the intermediate saddle, the saddle has to bear a load larger than that of the table body during the feed operation. As a result, the precision of the positioning operation in the X-axis direction is inferior to that in the Y-axis direction owing to the fact that the influence of inertia becomes larger when feeding the intermediate saddle than the table body.
The requirement of the location of a space for mounting the intermediate saddle or the like between the base and the table body increases the height of the table in the entirety. Hence the center of gravity is shifted to a higher position, thereby adversely affecting on the stability of feed. Furthermore, the rotary motors partially project from the side of the table, and thus, the overall size of the table is increased.
The prior art table further involves the following disadvantages. The necessity for a mechanism capable of converting the rotary motion of the rotary motor into linear motion causes such problems as an increase in the number of parts required, complexity of the structure, and troublesome works in assembly. If the table is frequently actuated, the ball screw and nut assemblies are heated, thereby lowering the precision of each of the ball screw and nut assemblies under the influence of thermal expansion.